Dinuclear lanthanide(III)/zinc(II) complexes with methyl 2-pyridyl ketone oxime

Abstract

The first use of methyl 2-pyridyl ketone oxime (mpkoH) in zinc(II)/lanthanide(III) chemistry leads to the [ZnLn(mpko)₃(mpkoH)₃](ClO₄)₂ and [ZnLn(NO₃)₂(mpko)₃(mpkoH)] families of dinuclear Zn^IILn^III complexes displaying blue-green, ligand-based photoluminescence; the Zn^IIDy^III compound shows field-induced relaxation of magnetization.

---

Dinuclear and polynuclear M^{II or III}/Ln^III coordination cluster complexes, where M^{II or III} are paramagnetic 3d-metal ions and Ln^III is a trivalent lanthanide ion,¹ occupy a unique place among mixed-metal molecular materials as a result of the interaction between 3d and 4f electron systems giving rise, for example, to alternatives² to homometallic 3d-metal Single-Molecule Magnets (SMMs)³ and magnetic refrigerants.⁴ An important feature here is the fact that 4f metal ions can contribute a large spin and, for most 4f ions, also the magnetic anisotropy needed for SMM behaviour. Furthermore, the coupling between 3d and 4f metal ions can be relatively strong in terms of superexchange interaction.

However, in contrast to the plethora of studies concerning discrete 3d/4f-metal complexes, where both metal ions are paramagnetic, there is rather limited information on complexes containing Zn^II (a diamagnetic 3d¹⁰ metal ion) and paramagnetic Ln^III centres.⁵ Such complexes are extremely useful because (i) they can help scientists to elucidate the Ln^III⋯Ln^III magnetic exchange interactions in a series of isostructural M^II_xLn^III_y coordination clusters (M = Mn, Fe, Co, Ni, Cu; y ≥ 2)^1c,6 and (ii) they often exhibit interesting photoluminescence properties and phenomena⁷ distinctly different from those of analogous complexes containing only Ln^III ions.

From a synthetic inorganic chemistry viewpoint, methods must be devised to combine 3d- and 4f-metal ions within a dinuclear or polynuclear molecule. One of our preferred routes is a “one-pot” procedure involving a mixture of 3d- and 4f-metal starting materials and an organic ligand possessing distinct functionalities, or “pockets”, for preferential bonding of the 3d and 4f ions. For example, the various anionic 2-pyridylmonoximes (Scheme 1) have been widely used to date in the synthesis of structurally and magnetically interesting 3d-,⁸ 3d/3d′- ⁹ and 3d/4f-metal¹⁰ complexes [M^II = paramagnetic 3d-metal ion]. However, there are no reports of their use in Zn^II/Ln^III chemistry. These ligands are, in fact, attractive for Zn^II/Ln^III chemistry because the hard, deprotonated O atom will favour binding to strongly oxophilic Ln^III ions, whereas the softer 2-pyridyl and oximate N atoms will favour coordination to the Zn^II centre.

Scheme 1 General structural formula of the family of 2-pyridylmonoximes (R = H, Me, Ph, 2-pyridyl, CN, NH₂, etc.). The ligand used in this work is methyl 2-pyridyl ketone oxime (R = Me), abbreviated as mpkoH.

We have been recently involved in a new research programme aiming to prepare, characterize and study discrete (i.e. non-polymeric), mixed Zn^II/Ln^III coordination cluster complexes with 2-pyridylmonoximate (Scheme 1) and 2,6-pyridylbisoximate bridging ligands. Our short-term goal is to establish routes for such complexes and to isolate the maximum number of products from a given ligand. Our longer-term goal is to force the carefully designed Zn^II-ligand moiety of the heterometallic complex to act as an efficient sensitizer, and one that is more efficient than the organic-only chromophore in exciting Ln^III ions (mainly Eu^III, Tb^III and Dy^III) for emission in the visible region of the spectrum.¹¹ It is also well known¹² that highly luminescent Ln^III complexes are of interest for a wide variety of photonic applications such as planar waveguide amplifiers, light-emitting diodes and bio-inspired luminescent probes. Most of the electronic transitions of the Ln^III ions involve a redistribution of electrons within the 4f sub-shell. Electric dipole selection rules forbid such transitions but these rules are relaxed by several mechanisms, such as coupling with vibrational states, J-mixing and mixing with opposite-parity wave functions (5d orbitals, ligand orbitals or charge transfer states). The coupling between these vibrational and electronic states and the 4f wavefunctions depends on the strength of the interaction between the 4f orbitals and the surrounding ligands; in view of the shielding of the 4f orbitals, the degree of mixing remains small, and so are the oscillator strengths of the f–f transitions.^12b As a consequence, even if many Ln^III compounds display a good quantum yield, direct excitation of the Ln^III ions rarely yields highly luminescent materials. This disadvantage may be overcome by employing suitable organic^12,13 or d-block¹⁴ (making use of fully-allowed, low-energy charge transfer transitions from p-character systems to d-character systems, e.g. luminescent complexes of d⁶ and d⁸ metal ions) chromophores as antenna groups to generate sensitised emission from Ln^III ions. In this communication we describe our preliminary efforts towards the realisation of the short-term goal mentioned above, i.e. to establish the chemistry of the Zn^II/Ln^III/2-pyridylmonoxime system.

Reactions of Zn(ClO₄)₂·6H₂O, mpkoH, Et₃N and Ln(NO₃)₃·6H₂O in a 1 : 6 : 3 : 2 molar ratio in MeOH at room temperature gave pale yellow (Ln = Eu, Gd, Tb) or pale orange (Ln = Dy, Ho) solutions, which upon storage at 15 °C gave pale yellow [ZnLn(mpko)₃(mpkoH)₃](ClO₄)₂·2H₂O (Ln = Eu, 1·2H₂O; Ln = Gd, 2·2H₂O; Ln = Tb, 3·2H₂O) and almost colourless [ZnLn(NO₃)₂(mpko)₃(mpkoH)] (Ln = Dy, 4; Ln = Ho, 5) complexes; typical yields were in the 50–60% range. The structures of 1·2H₂O and 4 were solved by single-crystal X-ray crystallography,‡ while the identity of 2·2H₂O, 3·2H₂O and 5 was confirmed by unit cell determination, IR spectroscopy and elemental (C, H, N) analyses.

Complex 1·2H₂O crystallizes in the triclinic space group P1̄. Its structure consists of dinuclear cations [ZnEu(mpko)₃(mpkoH)₃]²⁺ (Fig. 1), ClO₄⁻ anions and solvate H₂O molecules. The metal ions are bridged by the three oximate groups of the η¹:η¹:η¹:μ mpko⁻ ligands. The Zn^II ion is coordinated by six nitrogen atoms belonging to the “chelate” part of these ligands. This metal ion has a facial distorted octahedral geometry, the trans coordination angles being in the range 155.2(3)–158.4(3)°. The Eu^III centre is bound to an O₃N₆ set of donor atoms. The oxygen atoms (O4, O5, O6) belong to the deprotonated oximate groups of the {Zn(mpko)₃}⁻ unit, while the six nitrogen atoms belong to the three bidentate chelating (η¹:η¹) mpkoH ligands. Using the Continuous Shape Measure (CShM) approach,¹⁵ the coordination geometry of Eu^III can be best described as spherical capped square antiprismatic (Fig. S1†), the capping atom being N5; since the CShM value for this geometry (1.80735) is very close to that for the spherical tricapped trigonal prismatic geometry (1.80967), the Eu^III ion can also be considered as having the latter. Most 9-coordinate metal ions possessing three 5-membered chelating rings and three monodentate ligands (the three monodentate “ligands” for Eu are the terminally coordinated oximate oxygen atoms O4, O5 and O6) form polyhedra that appear very close to the interconversion path between the capped square antiprism and tricapped trigonal prism.¹⁵

Fig. 1 Partially labelled plot of the cation that is present in the structure of 1·2H₂O. Most H atoms have been omitted for clarity. Selected interatomic distances (Å) and angles (°): Eu⋯Zn 4.028(5), Eu–N 2.603(8)–2.728(7), Eu–O 2.313(6)–2.354(6), Zn–N 2.104(8)–2.196(7); N1–Eu–N2 59.1(2), N9–Zn–N10 74.6(3).

There are three strong intramolecular (intracationic) hydrogen bonds with uncoordinated oxime oxygens (O1, O2, O3) as donors and the coordinated oximate oxygens (O6, O4 and O5, respectively) as acceptors. There are also H-bonded parallelograms of the type HOH⋯O(ClO₂)O⋯HOH⋯O(ClO₂)O (Fig. S2†) and single HOH⋯O(ClO₃) H bonds in the crystal structure of the complex, which is further stabilized by weak intermolecular π–π stacking interactions and C–H⋯π interactions to form 2D honeycomb layers parallel to the plane formed by c and the bisection line of the a0b.

Complex 4 crystallizes in the monoclinic space group Cc. Its structure consists of dinuclear molecules [ZnDy(NO₃)₂(mpko)₃(mpkoH)]. The molecular structure of 4 is similar to that of the cation [ZnEu(mpko)₃(mpkoH)₃]²⁺, the main difference being the replacement of two N,N′-bidentate chelating mpkoH ligands of the latter with two bidentate chelating nitrato groups in the former (Fig. 2). This replacement gives neutral molecules (and not cations) in 4 and the Dy^III centre is thus bound to an O₇N₂ donor set. The coordination polyhedron of Dy^III can be described as a spherical capped square antiprism (Fig. S3†) with the oxime nitrogen N8 as the capping atom, although descriptions as a spherical tricapped trigonal prism and muffin are equally acceptable. Again there is evidence for a strong intramolecular O4–H(O4)⋯O3 H bond; two weak C–H⋯π interactions (Fig. S4†) extend the structure into two dimensions forming layers parallel to the ab plane and giving a sql topology.

Fig. 2 Partially labeled plot of the molecule that is present in the structure of 4. Only the H atom of the neutral oxime group of the unique mpkoH ligand is shown. Selected interatomic distances (Å) and angles (°): Dy⋯Zn 3.954(2), Dy–O(nitrato) 2.445(3)–2.566(2), Dy–O(oximato) 2.245(2)–2.343(2), Dy–N7 2.673(3), Dy–N8 2.535(3), Zn–N 2.103(2)–2.247(2); O8–Dy–O9 51.2(1), N7–Dy–N8 60.8(1), N1–Zn–N2 74.5(1).

The lanthanide(III)–N(mpkoH) and –O(oximato) bond lengths around Eu^III and Dy^III in 1·2H₂O and 4 show the expected tendency towards shorter values in the latter in line with the lanthanide(III) contraction. Compounds 1·2H₂O and 4 are the first structurally characterized heterometallic Zn^II/Ln^III complexes with 2-pyridyloxime/oximate ligation.

Upon maximum excitation at 400 nm, solid 4 displays photoluminescence at 448, 483 and 540 nm at room temperature (Fig. 3). The most probable origin of emission is ligand-based as the excitation and emission spectra of both mpkoH and 4 are observed in the same region. Thus, somewhat to our disappointment, no significant Dy^III emission was detected. Almost identical excitation and emission spectra are observed for solid samples of 1·2H₂O and 3·2H₂O (Fig. S5 and S6†), supporting our view that no Ln^III emission appears. Although not desirable, this is not an unusual situation for lanthanide(III) complexes with certain types of organic ligands.¹⁶ In our case, this means that the energy transfer from the organic ligands to Ln^III is not efficient, probably because the energy levels of the excited states of these ions lie higher than that of the excited state of the {Zn-mpko⁻}moiety; this requires a change of the ligand in the Zn^II moiety of the complex.

Fig. 3 Solid-state room-temperature excitation (emission at 540 nm, left) and emission (excitation at 400 nm, right) of mpkoH (dotted curves) and complex 4 (solid curves).

The room temperature χ_MT product for 4 (14.30 cm³ K mol⁻¹) under an applied dc field of 1000 Oe is consistent with the expected value of 14.17 cm³ K mol⁻¹ for one isolated Dy^III ion (⁶H_15/2 free ion, S = 5/2, L = 5, g_J = 4/3). The product decreases slowly to a value of 12.58 cm³ K mol⁻¹ at 2.2 K, before a small upturn at 2.0 K (Fig. 4, left). The decrease in χ_MT is due to the progressive depopulation of the Dy^III excited-state Stark sublevels.¹⁶ The small upturn below 2 K could be due to a weak ferromagnetic interaction between the complexes.¹⁷ The field dependence of magnetization shows that the magnetization reaches 5.4 μ_B at 70 kOe after a rapid increase at low fields (Fig. 4, right). The observed non-saturated magnetization at the highest field is much lower than the expected 10 μ_B for the Dy^III ion indicating some anisotropy in the system.^17,18

Fig. 4 Temperature dependence of χ_MT for 4 (left) and molar magnetization vs. field at indicated temperatures (right, the solid lines are guides for the eyes).

The dynamic magnetic properties of 4 were probed using ac susceptometry. Practically no signals for the out-of-phase component of the ac susceptibility were observed in the absence of a dc field at 1.8 K (Fig. S7†). However, an intense signal is observed with the application of an external dc field of 1000 Oe. Thus, on application of this field the positions of the maxima of the out-of-phase signals become strongly frequency-dependent (Fig. 5) as expected for a Single-Ion Magnet (SIM). The application of this field suppresses fast zero-field tunnelling of the magnetization, which is a well-documented behaviour for Ln^III-based SIMs and SMMs.¹⁷ To calculate the characteristic time and the barrier to relaxation of 4, the relaxation times were fitted with the frequencies occurring at the χ′′_max of the frequency-dependent ac susceptibility data (Fig. 5, left) by using the Orbach thermally activated relaxation law τ = τ₀ exp(U_eff/k_BT). Linear data following this law were only obtained between 4 K and 5.5 K (solid line, Fig. 5, right), with an effective energy barrier of U_eff = 33.3 K and τ₀ = 2.0 × 10⁻⁷ s. This suggests that the relaxation might follow a quantum regime below 4 K or that there is more than one thermally activated relaxation process in the complex. Thus, complex 4 can be termed as an “emissive field-induced SMM/SIM”.^1c,19 The observation of more than one well-resolved pathway for magnetic relaxation remains a rarity among mononuclear SMMs.²⁰

Fig. 5 Frequency dependence of the out-of-phase component of the ac susceptibility under an external dc field of 1000 Oe at the indicated temperatures (left) and the relaxation time of 4 as a function of temperature (dots) plotted against a thermally activated Arrhenius law (solid line) (right).

Conclusions

In conclusion, the members of the first two families of Zn^II/Ln^III/2-pyridylmonoximate complexes have been prepared. The complexes exhibit ligand-based blue-green photoluminescence, while Zn^II/Dy^III is considered to be a field-induced single-ion magnet. We are currently trying to investigate the possible presence of members of a third family of products in the Ln(NO₃)₃·xH₂O/Zn(ClO₄)₂·xH₂O/mpkoH general reaction system (we have strong evidence for this). Work is also in progress to enhance the aromatic content of the dinuclear Zn^IILn^III complexes by employing anionic phenyl 2-pyridine ketone oxime (R = Ph in Scheme 1) as a bridging ligand and PhCO₂⁻ (instead of NO₃⁻ or mpkoH) as terminal groups, with the hope of achieving efficient energy transfer from the Zn^II-ligand moiety to the lanthanide and “switch on” Ln^III-based emission. Achieving Dy^III-based emission in a Zn^IIDy^III SIM would enable us to correlate luminescence and magnetism^20a,21 since, in theory, the highest energy f–f transitions in the emission spectra of mononuclear Dy^III SIMs can be modelled to provide a direct picture of the splitting of the ground J multiplet.

Acknowledgements

This work was supported by the Alexander von Humboldt Foundation (postdoctoral research fellowship to KFK) and the DFG through SFB TR88 “3MET” (AKP). CDP gratefully acknowledges the Alexander Onassis Public Benefit Foundation for a PhD fellowship (G ZG 034-2/2012-2013).

Notes and references

1. For review-type articles, see: (a) C. D. Polyzou, C. G. Efthymiou, A. Escuer, L. Cunha-Silva, C. Papatriantafyllopoulou and S. P. Perlepes, Pure Appl. Chem., 2013, 85, 315; (b) J. W. Sharples and D. Collison, Coord. Chem. Rev., 2014, 260, 1; (c) H. L. C. Feltham and S. Brooker, Coord. Chem. Rev., 2014, 276, 1.
2. For example, see: (a) C. G. Efthymiou, Th. C. Stamatatos, C. Papatriantafyllopoulou, A. J. Tasiopoulos, W. Wernsdorfer, S. P. Perlepes and G. Christou, Inorg. Chem., 2010, 49, 9737; (b) S. Schmidt, D. Prodius, V. Mereacre, G. E. Kostakis and A. K. Powell, Chem. Commun., 2013, 49, 1696; (c) K. Chandra Mondal, A. Sundt, Y. Lan, G. E. Kostakis, O. Waldmann, L. Ungur, L. F. Chibotaru, C. E. Anson and A. K. Powell, Angew. Chem., Int. Ed., 2012, 51, 7550; (d) G. J. Sopasis, A. B. Canaj, A. Philippidis, M. Siczek, T. Lis, J. R. O'Brien, M. M. Antonakis, S. Pergantis and C. J. Milios, Inorg. Chem., 2012, 51, 5911; (e) D. Dermitzaki, G. Lorusso, C. P. Raptopoulou, V. Psycharis, A. Escuer, M. Evangelisti, S. P. Perlepes and Th. C. Stamatatos, Inorg. Chem., 2013, 52, 10235.
3. C. J. Milios and R. E. P. Winpenny, Struct. Bonding, 2015, 164, 1.
4. M. Evangelisti and E. K. Brechin, Dalton Trans., 2010, 39, 4672.
5. For example, see: (a) W.-K. Wong, X. Yang, R. A. Jones, J. H. Rivers, V. Lynch, W.-K. Lo, D. Xiao, M. M. Oye and A. L. Holmes, Inorg. Chem., 2006, 45, 4340; (b) S. Akine, F. Utsuno, T. Taniguchi and T. Nabeshima, Eur. J. Inorg. Chem., 2010, 3143.
6. W.-R. Yu, G.-H. Lee and E.-C. Yang, Dalton Trans., 2013, 42, 3941.
7. (a) C. E. Burrow, T. J. Burchell, P.-H. Lin, F. Habib, W. Wernsdorfer, R. Clerac and M. Murugesu, Inorg. Chem., 2009, 48, 8051; (b) T. D. Pasatoiu, A. M. Madalan, M. Zamfirescu, C. Tiseanu and M. Andruh, Phys. Chem. Chem. Phys., 2012, 14, 11448.
8. K. F. Konidaris, V. Bekiari, E. Katsoulakou, C. P. Raptopoulou, V. Psycharis, E. Manessi-Zoupa, G. E. Kostakis and S. P. Perlepes, Dalton Trans., 2012, 41, 3797.
9. S. Ross, T. Weyhermuller, E. Bill, K. Wieghardt and P. Chaudhuri, Inorg. Chem., 2001, 40, 6656.
10. C. D. Polyzou, H. Nikolaou, C. Papatriantafyllopoulou, V. Psycharis, A. Terzis, C. P. Raptopoulou, A. Escuer and S. P. Perlepes, Dalton Trans., 2012, 41, 13755.
11. (a) T. D. Pasatoiu, A. M. Madalan, M. U. Kumke, C. Tiseanu and M. Andruh, Inorg. Chem., 2010, 49, 2310; (b) T. D. Pasatoiu, C. Tiseanu, A. M. Madalan, B. Jurca, C. Duhayon, J. P. Sutter and M. Andruh, Inorg. Chem., 2011, 50, 5879; (c) M. A. Palacios, S. Titos-Padilla, J. Ruiz, J. M. Herrera, S. T. A. Pope, E. K. Brechin and E. Colacio, Inorg. Chem., 2014, 53, 1465.
12. (a) J.-C. Bunzli, Acc. Chem. Res., 2006, 39, 53; (b) J.-C. G. Bűnzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048.
13. S. Swavey and R. Swavey, Coord. Chem. Rev., 2009, 253, 2627.
14. D. Sykes and M. D. Ward, Chem. Commun., 2011, 47, 2279.
15. A. Ruiz-Martinez, D. Casanova and S. Alvarez, Chem. – Eur. J., 2008, 14, 1291.
16. N. C. Anastasiadis, C. M. Granadeiro, N. Klouras, L. Cunha-Silva, C. P. Raptopoulou, V. Psycharis, V. Bekiari, S. S. Balula, A. Escuer and S. P. Perlepes, Inorg. Chem., 2013, 52, 4145.
17. H. L. C. Feltham, Y. Lan, F. Klower, L. Ungur, L. F. Chibotaru, A. K. Powell and S. Brooker, Chem. – Eur. J., 2011, 17, 4362.
18. M. Ren, D. Pinkowicz, M. Yoon, K. Kim, L.-M. Zheng, B. K. Breedlove and M. Yamashita, Inorg. Chem., 2013, 52, 8342.
19. As has been suggested,^1c the term “field-induced SMM” appears to be misleading as such compounds are not magnets as they show no slow relaxation (and no hysteresis) in the absence of a field.
20. (a) J. Long, J. Rouquette, J.-M. Thibaud, R. A. S. Ferreira, L. D. Carlos, B. Donnadieu, V. Vieru, L. F. Chibotaru, L. Konczewicz, J. Haines, Y. Guari and J. Larionova, Angew. Chem., Int. Ed., 2015, 54, 2236; (b) A. Watanabe, A. Yamashita, M. Nakano, T. Yammura and T. Kajiwara, Chem. – Eur. J., 2011, 17, 7428; (c) M. Jeletic, P.-H. Lin, J. J. Le Roy, I. Korobkov, S. I. Gorelsky and M. Murugesu, J. Am. Chem. Soc., 2011, 133, 19286; (d) J. Ruiz, A. J. Mota, A. Rodriguez-Diéguez, S. Titos, J. M. Herrera, E. Ruiz, E. Cremades, J. P. Costes and E. Colacio, Chem. Commun., 2012, 48, 7916.
21. (a) J. Long, R. Vallat, R. A. S. Ferreira, L. D. Carlos, F. A. Almeida Paz, Y. Guari and J. Larionova, Chem. Commun., 2012, 48, 9974; (b) G. Cucinotta, M. Perfetti, J. Luzon, M. Etiene, P.-E. Car, A. Caneschi, G. Calvez, K. Bernot and R. Sessoli, Angew. Chem., Int. Ed., 2012, 51, 1606.

---

Footnotes

† Electronic supplementary information (ESI) available: Full experimental details of the synthesis of complexes 1·2H₂O and 4, analytical data for all complexes, short experimental for single-crystal X-ray crystallography, metric parameters for the H bonds in 1·2H₂O, and structural (Fig. S1–S4), optical (Fig. S5 and S6) and magnetic (Fig. S7 and S8) plots. CCDC 1021513 (1·2H₂O) and 1021514 (4). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt03663a

‡ Crystallographic data for complex 1·2H₂O: C₄₂H₄₉EuZnN₁₂O₁₆Cl₂, M_r = 1266.16, triclinic, P1̄, a = 13.8641(15) Å, b = 14.5627(19) Å, c = 15.190(2) Å, α = 96.519(11)°, β = 113.081(10)°, γ = 109.850(9)°, V = 2543.6(6) ų, Z = 2, D_c = 1.653 g cm⁻³, μ (Mo Kα, λ = 0.71073 Å) = 1.878 mm⁻¹, T = 180(2) K, 17 067 reflections collected, 10 653 unique (R_int = 0.0919), R₁ on F (wR₂ on F²) = 0.0606 (0.1164) for 4678 observed (I > 2σ(I)) reflections. Crystallographic data for 4: C₂₈H₂₉DyZnN₁₀O₁₀, M_r = 893.48, monoclinic, Cc, a = 15.9387(11) Å, b = 9.3158(4) Å, c = 22.5168(17) Å, β = 94.590(6)°, V = 3332.6(4) ų, Z = 4, D_c = 1.781 g cm⁻³, μ (Mo Kα, λ = 0.71073 Å) = 3.015 mm⁻¹, T = 180(2) K, 11 098 reflections collected, 6259 unique (R_int = 0.0265), R₁ on F (wR₂ on F²) = 0.0191 (0.0480) for 6209 observed (I > 2σ(I)) reflections. Unit cell information: complex 2·2H₂O: triclinic, P1̄, a = 13.8894 Å, b = 14.6063 Å, c = 15.2384 Å, α = 96.985°, β = 112.719°, γ = 109.824°, V = 2566.45 ų. Complex 3·2H₂O: triclinic, P1̄, a = 13.8574 Å, b = 14.5554 Å, c = 15.1713 Å, α = 96.684°, β = 112.834°, γ = 109.960°, V = 2539.08 ų. Complex 5: monoclinic, Cc, a = 15.9450 Å, b = 9.3192 Å, c = 22.499 Å, β = 95.061°, V = 3330.2 ų. CCDC numbers 1021513 (1·2H₂O) and 1021514 (4).

---